Piston thermal management presents continuing challenges to piston integrity due to increasing loads demanded for modern engines. In a typical piston, four areas are particularly susceptible to thermal damage: the piston crown, the ring grooves, the piston/wristpin interface, and the piston undercrown. If combustion temperatures felt by the crown end surface exceed the oxidation temperature of the crown materials, oxidation can result. The crown may be subject to mechanical failure caused by stress/fatigue at the oxidized sites. The piston's rings, ring grooves, and lands may exhibit carbon build-up due to lubricating oil being heated above its coking temperature. A hot wristpin bore can result in lower load-carrying capacity of the piston bearing. As with the ring grooves, the piston undercrown may also be subject to oil coking.
In some aspects of opposed-piston combustion chamber construction it is desirable to utilize pistons whose crowns include highly contoured end surfaces which produce complex, turbulent charge air motion that encourages uniform mixing of air and fuel. An example of a highly contoured piston end surface that forms a combustion chamber with an oppositely-disposed, similarly-contoured piston end surface is shown in FIG. 11 of US 2011/0271932 A1. Combustion imposes a heavy thermal load on these pistons. Their highly contoured end surfaces create non-uniform thermal profiles with concentrations of heat (“hot spots”) that can lead to asymmetrical thermal stress, wear, and piston fracture.
Typically, three approaches are taken to manage piston temperatures. In one, high thermal resistance of the piston crown reduces or blocks the passage of heat from the combustion chamber into the crown. A second approach relies on conduction of heat from the crown to the cylinder bore through the rings, ring grooves, lands, and skirt of the piston. The third approach uses a flow of liquid coolant to remove heat from the undercrown. Modern piston constructions typically include all three approaches.
Liquid coolant is typically applied to the undercrown by means of galleries and/or nozzles. For example, U.S. Pat. No. 8,430,070 teaches a piston cooling construction including an outer gallery that receives and transports oil for cooling the piston undercrown. An oil outlet is provided on the bottom of the outer gallery. A nozzle mounted to the floor of the outer gallery, in fluid communication with the oil outlet is aimed toward the undercrown. Oil is inertially pumped from the gallery through the oil outlet in response to upward movement of the piston. The pumped oil is sprayed from the nozzle onto the undercrown in response to upward movement of the piston.
An example of undercrown cooling in an opposed-piston context is shown in FIG. 5 of the Applicant's US 2012/0073526 A1 wherein a piston with a contoured end surface includes an annular gallery 256 within the crown that follows the periphery of the crown, underneath the end surface. The annular gallery is in fluid communication with a central gallery 257 underneath the central portion of the end surface. A nozzle 262, separate from the piston, is aimed at an opening in the annular gallery 256. A high velocity jet of oil emitted by the nozzle 262 travels into the annular gallery, striking a specific portion of the crown underneath a ridge of the end surface that bears a heavy thermal burden during combustion. The jet cools the specific crown portion by impingement. The oil then flows through the annular and central galleries, thereby cooling additional portions of the undercrown. Oil flows out of the central gallery and exits the piston.
The cooling capability of the nozzle described in U.S. Pat. No. 8,430,070 is limited by the inertial pumping operation which occurs only during upward movement of the piston. As a result, the undercrown is cooled by spraying oil through only one half of the piston's operational cycle. Furthermore, because the sprayed oil is obtained from the cooling gallery, it is already heated, which limits its cooling capacity when emitted by the nozzle. The cooling construction of US 2012/0073526 A1 brings oil into the piston via a nozzle external to the piston. Separate transport channels are required to bring up pressurized oil to cool the undercrown and to lubricate the piston rod coupling mechanism. As a result, oil is provided throughout the operating cycle of the piston, but at the penalty of increased complexity and cost of the lubrication system.
Accordingly, there is a need for delivering lubricating oil to a piston for cooling the undercrown in a manner that maintains the flow of lubricating oil throughout the piston's cycle of operation without adding to the complexity and cost of the system that transports the oil to the piston for lubrication.